The present invention relates to a method or apparatus for canceling echoes in the system using plural received signals and a single or plural transmission signals.
There are multi-channel echo canceling methods or apparatuses that remove echoes caused by received signals propagating through spatial acoustic paths in the system of transmitting plural received signals or a single or plural transmission signals. Two types of multi-channel echo canceling apparatuses including a cascade connection type and a linear coupled type have been proposed in the Technical Report of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan, Vol. 84, No. 330, pp. 7-14, CS-84-178 (hereinafter referred to as reference 1). According to the reference (1), the cascade connection type is inferior to the linear coupled type in echo suppression capability caused by the constitutional constraint. Here, the case where a linear coupled multi-channel echo canceling apparatus (echo canceller) is applied to a two-channel system, together with received signals and transmission signals, will be described below as a prior art.
FIG. 24 shows a linear coupled multi-channel echo canceller. The first loudspeaker 3 reproduces the first received signal 1. The second loudspeaker 4 reproduces the second received signal 2. The first echo 5 which is generated as the signal propagating via spatial acoustic paths from the first loudspeaker 3 to the first microphone 9 and the second echo 6 which is produced as the signal propagating through spatial acoustic paths from the second loudspeaker 4 to the first microphone 9 are added to the first transmission signal 12 or the voice of the talker 11 received with the first microphone 9, so that a first mixed signal 14 is created. Similarly, the third echo 7 which is generated as the signal propagating the spatial acoustic paths from the first loudspeaker 3 to the second microphone 10 and the fourth echo 8 which is produced as the signal propagating through spatial acoustic paths from the second loudspeaker 4 to the second microphone 10 are added to the second transmission signal 13 or the voice of the talker 11 received with the second microphone 10, so that a second mixed signal 15 is created.
In order to remove the echoes introduced into the first mixed signal 14, the first adaptive filter 121 receives the first received signal 1 and then creates an echo replica 125 corresponding to the first echo 5. The second adaptive filter 122 receives the second received signal 2 and then creates an echo replica 126 corresponding to the second echo 6. The first subtracter 129 subtracts the echo replicas 125 and 126 corresponding to the first echo 5 and the second echo 6, respectively, from the first mixed signal 14. The first and second adaptive filters 121 and 122 are controlled to minimize the output of the first subtracter 129. The output of the first subtracter 129 becomes the first output signal 16 of the echo canceller 100.
In order to remove the echoes introduced into the second mixed signal 15, the third adaptive filter 123 receives the first received signal 1 and then creates an echo replica 127 corresponding to the third echo 7. The fourth adaptive filter 124 receives the second received signal 2 and then creates an echo replica 128 corresponding to the fourth echo 8. The second subtracter 130 subtracts the replicas 127 and 128 corresponding to the third echo 7 and the fourth echo 8 from the second mixed signal 15, respectively. The third and fourth adaptive filters 123 and 124 are controlled to minimize the output of the second subtracter 130. The output of the second subtracter 130 becomes the second output signal 17 of the echo canceller 100.
In the multi-channel teleconferencing system being one of important applications for multi-channel echo cancellation, plural microphones receives voices of talkers. Hence, received signals by each microphone can be approximately regarded as signals with an attenuation and a delay each depending on the distance between the talker and the microphone. The mutual correlation between received signals in different channels becomes very high. Let us now assume that the second received signal 2 which was generated as a delayed version of the first received signal 1, an echo path which can be modeled as a finite impulse response filter, and an echo canceller based on linear combination with adaptive finite impulse response filters.
It is now assumed that x1(n) is a first received signal 1 at the time n, x2(n) is a second received signal 2 at the time n, and d(n) is an echo introduced into the mixed signal 14. When the time differences between the first received signals 1 and the second received signals 2 is nd (natural number), X2(n) is expressed by:
x2(n)=x1(nxe2x88x92nd)xe2x80x83xe2x80x83(1)
For simplification, let us now assume that all the spatial acoustic paths ranging from the first and second loudspeakers 3 and 4 to the first and second microphones 9 and 10 are equal in the impulse response length (N). The echo d(n) mixed in the mixed signal 14 is the sum of the echo 5 and the echo 6 and is expressed by:                                           d            ⁡                          (              n              )                                =                                    ∑                              i                =                0                                            N                -                1                                      ⁢                          h              1                                      ⁢                  ,          i                ⁢                                            x              1                        ⁡                          (                              n                -                i                            )                                +                                    ∑                              i                =                0                                            N                -                1                                      ⁢                          h              2                                      ⁢                  ,          i                ⁢                              x            2                    ⁡                      (                          n              -              i                        )                                              (        2        )            
where h1,j is an impulse response sample value of a spatial acoustic path ranging from the loudspeaker 3 to the microphone 9, h2,i is an impulse response sample value of a spatial acoustic path ranging from the loudspeaker 4 to the microphone 9, and i is an integer value between 0 and Nxe2x88x921.
When x2(n) is eliminated by substituting the equation (1) into (2), the following equation (3) is obtained:                               d          ⁡                      (            n            )                          =                                            ∑                              i                =                0                                                              n                  d                                -                1                                      ⁢                                          h                                  1                  ,                  i                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                                  n                  d                                                            N                -                1                                      ⁢                                          (                                                      h                                          1                      ,                      i                                                        +                                      h                                          2                      ,                                              i                        -                                                  n                          d                                                                                                                    )                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    1                                    )                                                              +                                    ∑                              i                =                                  N                  -                                      n                    d                                                                              N                -                1                                      ⁢                                          h                                  2                  ,                  i                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                                          n                      d                                        -                    i                                    )                                                                                        (        3        )            
The echo replica d(n) hat created by each of the adaptive filters 121 and 122 is expressed by:                                           d            ^                    ⁡                      (            n            )                          =                                            ∑                              i                =                0                                            N                -                1                                      ⁢                                                            w                                      1                    ,                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                                            w                                      2                    ,                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                x                  2                                ⁡                                  (                                      n                    -                    i                                    )                                                                                        (        4        )            
where w1,i(n) is the i-th filter coefficient of the adaptive filter 121 and w2,i(n) is the i-th filter coefficient of the adaptive filter 122.
When x2(n) is eliminated by substituting the equation (1) into the equation (4), the following equation (5) is obtained:                                           d            ^                    ⁡                      (            n            )                          =                                            ∑                              i                =                0                                                              n                  d                                -                1                                      ⁢                                                            w                                      1                    ,                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                                  n                  d                                                            N                -                1                                      ⁢                                          {                                                                            w                                              1                        ,                        i                                                              ⁡                                          (                      n                      )                                                        +                                                            w                                              2                        ,                                                  i                          -                                                      n                            d                                                                                                                ⁡                                          (                      n                      )                                                                      }                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                    i                                    )                                                              +                                    ∑                              i                =                                  N                  -                                      n                    d                                                                              N                -                1                                      ⁢                                                            w                                      2                    ,                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                x                  1                                ⁡                                  (                                      n                    -                                          n                      d                                        -                    i                                    )                                                                                        (        5        )            
The residual echo e(n) is expressed by:                                                                         e                ⁡                                  (                  n                  )                                            =                              xe2x80x83                            ⁢                                                d                  ⁡                                      (                    n                    )                                                  -                                                      d                    ^                                    ⁡                                      (                    n                    )                                                                                                                          =                              xe2x80x83                            ⁢                                                                    ∑                                          i                      =                      0                                                                                      n                        d                                            -                      1                                                        ⁢                                                            {                                                                        h                                                      1                            ,                            i                                                                          -                                                                              w                                                          1                              ,                              i                                                                                ⁡                                                      (                            n                            )                                                                                              }                                        ⁢                                                                  x                        1                                            ⁡                                              (                                                  n                          -                          i                                                )                                                                                            +                                                                                                        xe2x80x83                            ⁢                                                                    ∑                                          i                      =                                              n                        d                                                                                    N                      -                      1                                                        ⁢                                                            {                                                                        h                                                      1                            ,                            i                                                                          +                                                  h                                                      2                            ,                                                          i                              -                                                              n                                d                                                                                                                                    -                                                                              w                                                          1                              ,                              i                                                                                ⁡                                                      (                            n                            )                                                                          -                                                                              w                                                          2                              ,                                                              i                                -                                                                  n                                  d                                                                                                                                              ⁡                                                      (                            n                            )                                                                                              }                                        ⁢                                                                  x                        1                                            ⁡                                              (                                                  n                          -                          i                                                )                                                                                            +                                                                                                        xe2x80x83                            ⁢                                                ∑                                      i                    =                                          N                      -                                              n                        d                                                                                                  N                    -                    1                                                  ⁢                                                      {                                                                  h                                                  2                          ,                          i                                                                    -                                                                        w                                                      2                            ,                            i                                                                          ⁡                                                  (                          n                          )                                                                                      }                                    ⁢                                                            x                      1                                        ⁡                                          (                                              n                        -                                                  n                          d                                                -                        i                                            )                                                                                                                              (        6        )            
The condition of completely eliminating echoes from the equation (6) is expressed by:
xe2x80x83h1,i=w1,i(n) i=0, . . . ,ndxe2x88x921
h1,i+h2,ixe2x88x92nd=w1,i(n)+w2,ixe2x88x92nd(n) i=nd, . . . ,Nxe2x88x921
h2,i=w2,i(n) i=Nxe2x88x92nd, . . . ,Nxe2x88x921xe2x80x83xe2x80x83(7)
The following expression is uniquely determined by the equation (7).
w1,0(n), . . . ,w1xe2x88x92nxe2x88x921(n)and w2,Nxe2x88x92nd(n), . . . ,w2,Nxe2x88x921(n)xe2x80x83xe2x80x83[1]
However, it is clear that the following expression has an unlimited number of solutions.
i w1,nd(n), . . . ,w1,Nxe2x88x921(n) and w2,0, . . . ,w2,Nxe2x88x92ndxe2x88x921(n)[2]
Particularly the solutions of the following equation depend on values of nd.
w1,nd(n), . . . ,w1,Nxe2x88x921(n) and w2,0, . . . ,w2,Nxe2x88x92ndxe2x88x921(n)xe2x80x83xe2x80x83[3]
That is, a change of nd caused by a shift of the talker""s position leads to a change of the solution. This means that the echo canceling capability is degraded even with no variations in echo path. This creates an obstacle to applications in actual environments. In these discussions, the adaptive filters 121 and 122 used to remove echoes mixed in the mixed signal 14 have been assumed. Similarly, this discussion is applicable to adaptive filters 123 and 124.
In order to solve the above-mentioned problems, there is a multi-channel echo canceling method where adaptive filters, respectively corresponding one to one to mixed signals, create echo replicas in response to one received signal. Thus, one adaptive filter per channel can estimate echoes caused by signals propagating from a single sound source in plural paths. This method was proposed in the IEEE Proceedings of International Conference on Acoustics, Speech and Signal Processing, Vol. 2, 1994, pp. 245-248 (hereinafter referred to as reference 2).
The multi-channel echo canceling method of the reference 2 can avoid indefinite solutions because one adaptive filter removes the echo generated in one channel. As a result, the adaptive filter coefficients are converged to optimum values which are uniquely determined. However, the reference 2 discloses evaluation results proving the fact that the echo suppression is degraded when the parameters determined by the environment in use such as the locations of microphones receiving the taker""s voice are not within a certain range. Hence, a multi-channel echo canceller based on linear combination has to be used, by considering the use in a wide variety of environments.
Based on the above-mentioned premise, there is a method of uniquely determining the adaptive filter coefficients, for a multi-channel echo canceller based on linear combination, where the received signal is delayed and then is alternately selected as a new received signal the delayed signal in place of the received signal. This method was proposed in Technical Report, the Institute of Electronics, Information and Communication Engineers of Japan, Vol. DSP97-1, 1997, pp. 1-8 (hereinafter referred to as reference 3). In this multi-channel echo canceling method, the number of conditions used to calculate adaptive filter coefficients is increased because of the introduction of the delayed received signals, therefore, indefinite solutions do not occur. As a result, the adaptive filter coefficients converge to optimum values which are uniquely determined. However, shift of the acoustic image may often be perceived when the received signal is switched between the original and its delayed version. In order to overcome such a problem, there is a method of correcting the magnitudes of signals in both channels when the received signal and the delayed received signal are switched. This method was proposed in the Proceedings of the 12-th digital signal processing symposium of the Institute of Electronics, Information and Communication Engineers of Japan, 1997, pp. 531-536 (hereinafter referred the as reference 4). Moreover, there is a method that can uniquely determine adaptive filter coefficients by applying non-linear processing to the received signals in both channels, instead of switching the received signal and the delayed received signal. This method was proposed in the Proceedings of the IEEE International Conference on Acoustic, Speech and Signal Proceeding, Vol. 1, 1997, pp. 303-306 (hereinafter referred to as reference 5). However, the method disclosed in the reference 4 provides a slow convergence rate, compared with the multi-channel echo canceller based on linear combination. Moreover, the Proceeding of the IEEE International Conference on Acoustics, Speech and Signal Processing, Vol. 6, 1998, pp. 3677-3680 (hereinafter referred to as reference 6) discloses that the method in the reference 5 provides a slower convergence rate, compared with the method in the reference 4.
As described in detail by referring to FIG. 24, the conventional multi-channel echo canceling method or apparatus has the disadvantage in that the solutions to adaptive filter coefficients are indefinite and are not guaranteed to be equal to the solutions uniquely determined by the impulse responses in the echo paths. Moreover, in the method proposed in the reference 3, a shift of an acoustic (or sound) image may often be perceived when the received signal or the delayed received signal is selected. Moreover, in the methods proposed in the references 4 and 5, the convergence is slow, compared with the conventional multi-channel echo canceling method or apparatus.
The objective of the invention is to provide a multi-channel echo canceling method or apparatus with an excellent sound quality and a short convergence time, wherein the coefficient values of the adaptive filters are converged to correct values which are uniquely determined by the impulse responses of the echo paths.
In the multi-channel echo canceling apparatus according to the present invention, received signals are non-linearly processed while one obtained by delaying a received signal is used as a new received signal.
Specifically, the multi-channel echo canceling apparatus has magnitude processing circuits (605 and 606 in FIG. 1) for non-linearly processing received signals 1 and 2 and a delay processing circuit (200 in FIG. 1) for delaying the received signal 2 and then supplying the delayed signal to adaptive filters 122 and 124 and to a digital-to-analog converter (DAC) 19.
Furthermore, the multi-channel echo canceling apparatus non-linearly processes the received signals. The multi-channel echo canceling apparatus also uses a delayed received signal as a new received signal and applies magnitude correction to the other input signals.
Specifically, the multi-channel echo canceling apparatus has a magnitude processing circuit (605 in FIG. 17) for non-linearly processing the received signal 2, a delay processing circuit (300 in FIG. 17) for delaying a non-linearly processed signal and then supplying the delayed signal to the adaptive filters 122 and 124 and the digital-to-analog converter 19, a magnitude processing circuit (606 in FIG. 17) for non-linearly processing the received signal 1, and a magnitude compensating circuit (400 in FIG. 17) for correcting the magnitude of the non-linearly processed signal and then supplying the corrected signal to the adaptive filters 121 and 123 and the digital-to-analog converter 18.
In the multi-channel echo canceling apparatus or method according to the present invention, one received signal is filtered to create a supplement signal. The adaptive filter is operated using a new signal obtained by time-multiplexing an original received signal and the created supplement signal. A plurality of adaptive filters estimate echoes caused by propagating from one signal source in plural paths. For that reason, an increase in the number of conditions for solving adaptive filter coefficients makes solutions definite eliminating the existing problem of indefinite coefficients. Hence, the adaptive filter coefficients are converged to optimum values which are uniquely determined. Moreover, the multiplex parameters of the original received signal and the supplement signal are controlled based on the nature of the received signal while movement of the sound image caused by introducing the supplement signal is canceled in the magnitude correction process to the input signal. Hence, degradation of the sound quality of the audible received signal directly supplied to the loudspeaker is suppressed so that a good sound quality can be maintained. Moreover, the synergistic effect of introducing the non-linear processing and the supplement signal can shorten the convergence time.